This proposal is concerned with the role of neuropeptides in synaptic transmission between small diameter sensory afferents and nociceptive neurons in the dorsal horn of the spinal cord. Substance P, somatostatin and cholecystokinin are present within small diameter sensory afferents that project to the dorsal horn. Several lines of evidence now suggest that Substance P may be a transmitter released from the central terminals of some nociceptive afferents. Enkephalin-containing interneurons within the superficial dorsal horn are in close proximity to nociceptive afferent terminals that contain opiate receptors. Moreover, the spinal analgesic actions of opiates and opioid peptides may result in part from inhibition of substance P release from sensory afferent terminals. The precise relationship between enkephalin interneurons and peptide-containing afferent terminals in the dorsal horn is still unclear, largely as a result of the difficulty in obtaining stable intracellular recording from dorsal horn neurons. We intend to study the action of peptides on dorsal horn neurons, in a spinal cord slice preparation that retains a high degree of synaptic organization. Intracellular recordings will be made from neurons in horizontal spinal cord slices containing the superficial dorsal horn with the dorsal root intact, to enable afferent inputs to be activated. Our aim will be to record intracellularly from neurons in laminae I and II of the dorsal horn, to characterize the membrane properties of these neurons and to examine their sensitivity to iontophoretic or pressure application of peptides known to be present in primary afferent and dorsal horn neurons. We also intend to activate dorsal horn neurons by stimulation of afferent fibers in the dorsal root and to examine the involvement of substance P, opioid and other peptides in the processing and regulation of sensory transmission at primary afferent synapses. Intracellular markers (horseradish peroxidase, lucifer yellow) will be injected into cells in which the peptides sensitivity has been examined, in order to determine the location and dendritic arborization of these cells. Using antisera raised against substance P, somatostatin, cholecystokinin and enkephalin, dual color staining immunocytochemical techniques will be used to reveal the peptide content of neurons.